The invention relates to a particle filter in the form of a pleated non-woven layer. Filters of the generic type are used for instance as ambient air filters, and in particular as motor vehicle interior filters. During the manufacture of filters of the generic type, a material disposed in layers is pleated, i.e. it is arranged in concertina-type pleats, the distance of these pleats from each other being customarily fixed by a marginal strip glued on or by clamping in a frame. A filter of this design is then itself inserted for replacement in a filter frame.
Conventionally, various designs are available for the filtration of motor vehicle interior air:
On the one hand non-woven staple fibers are known, the pleatability of which is attained by a meshed support sheet for example of polypropylene. On the other hand there are so-called meltblowns which are placed on spunbonded fabrics or paper carriers and are united with same by ultrasonic spots or hot-melt adhesive spots.
The only type of filter which, in this field, does without any supporting layer is the so-called grit mat which is however flat and non-pleated or non-pleatable when inserted into the filter receptacle.
All the filter materials that have to be pleated have in common that they must exhibit a certain rigidity of their own in order for them to maintain their pleated design when employed in practice, which is why conventionally these non-wovens are laminated, i.e. they are combined with support layers.
The meltblowns and non-woven staple fibers mentioned either have too little substance or they are too soft to ensure by themselves proper and durable pleating.
Using support layers results in that, in as much as the cost effectiveness ratio is concerned, the costs of a spunbonded fabric for meltblown support can amount to as much as 70% of the total cost of the composite. The costs of a meshed sheet for the support of non-woven staple fibers amount to as much as 20%, the total cost being considerably higher than in the case of the design mentioned above.
The support components are of use exclusively in the sense of mechanical stabilization, whereas they are substantially useless from the point of view of filtering technology. Spunbonded fabrics only can retain particles in the range of &gt;5 .mu.m, which is why they are designated as prefilters. However, particles of this size deposit before reaching the heating or conditioning chamber for instance of a motor vehicle. Furthermore, a serious drawback resides in that any addition of non-woven or meshed sheet is always accompanied with an increased pressure drop of the entire laminated composite, which may amount to as much as 30%.
This is an important aspect since today's motor vehicles have an accurately calculated energy distribution system. Only restricted quantities of energy are available to the range of heating/venting/conditioning. The costs of vehicle components are allowed to range only within a close compass. On the other hand the demands of motor vehicle buyers for equipment convenience and safety are getting higher and higher. From these aspects, particle filters of as low a pressure drop as possible are of special importance, low pressure implying a ventilated motor of small dimensions and, therefore, correspondingly low energy consumption. Furthermore, low differential pressure also means little noise of the fan for the throughput of certain quantities of air in the vehicle and consequently increased convenience of driving.
The demand for filter systems of low differential pressure competes with the requested filtration capacity and requested service life, i.e. the time in terms of mileage, for which a filter may remain in a vehicle before it must be replaced.
Pollen filters which only filter pollen from the air streaming into a vehicle are of little help to allergic persons. The allergens which the immune system of these persons reacts to are proteins, the diameter of which is only a fraction of the pollen diameter. They are in the range of 0.1 .mu.m, i.e. in the most problematic range for particle filters, the so-called MPPS (most penetrating particle size). Consequently, solutions that are to be of some use to allergic persons must have a filtration capacity of at least 50%, this filtration capacity being measured by means of an aerosol, the particles of which have approximately the same density, for example NaCl. Incorporating filters of this type in motor vehicles is intended to attain a service life of 30,000 kilometers.
In theory it would be possible to produce non-woven staple fiber and melt-blown fabrics in such a way that they have sufficient rigidity of their own, thus being pleatable. However, it must be taken into consideration that for non-woven staple fiber fabrics stabilization takes place either thermally or by binders, it being necessary in the former case to accept a comparatively high differential pressure and in the latter case to accept very high emission rates. On the other hand, in order for meltblown fabrics to be rendered sufficiently stable to be used for instance in a motor vehicle interior filter box, a grammage or weight per unit area of more than 130 g/m.sup.2 would have to be attained. The diameter of a meltblown fiber is in the range of approximately 2 .mu.m. Given the grammage and fiber diameter, a non-acceptable differential pressure has to be expected.